A Low Earth Orbit (LEO) is a satellite orbit around Earth with an altitude between 160 kilometers (99 mi) (orbital period of about 88 minutes) and approximately 2,000 kilometers (1,200 mi) (orbital period of about 127 minutes). These orbits can be distinguished from a higher-altitude Geostationary Earth Orbit (GEO) in which a satellite remains above a fixed point on the Earth. Several large LEO satellite constellations are currently being developed. Many of these constellations utilize fixed beam patterns to provide spatial division and better link performance to subscriber terminals. The fixed beams are generated either by individual horns, or by the use of Butler matrices to operate the horns as a phased array antenna system.
Systems using fixed beams suffer from similar launch and operation issues. First, due to requirements for non-interference with GEO spacecraft, they must avoid transmitting and receiving directly up/down to ground equipment near the equator. Second, because these constellations must utilize many spacecraft for full coverage, it can take many months/years from launch of the first satellite to completion of the constellation. During this time, systems do not have full coverage, and so cannot begin paid operation, delaying the revenue stream. Third, the density of spacecraft increases in higher or lower latitudes for constellations with near-polar orbits, as the planes of such orbits move closer together near the poles. This results in increased beam overlap, which may result in increased interference since the spacecraft all use the same frequency channels over a particular region. This also means the constellation is less efficient than it could be over these regions.
The first issue (non-interference with GEO spacecraft signals) is solved by either disabling the links near the equator, or tilting the beams (possibly by tilting the spacecraft) in the in-track direction to provide coverage which is not in line with GEO spacecraft. For fixed beams, there is no solution for the second issue. Generation of revenue must wait until enough spacecraft are in orbit. For the third issue, beams may be turned off to conserve power, but the beam placement and coverage will not be ideal as the spacecraft moves through the latitudes. This means a loss of revenue, as the system serves fewer subscribers than it would if the beam pattern were optimized throughout the range of operating latitudes.
What is needed, therefore, are improved techniques for beamforming.